For a self-published author, buying ISBNs is OPTIONAL. Completely, entirely optional.

There are reasons to buy them, and they only make sense once your books start to gain traction.

For ebooks listed at retailers, they're entirely unnecessary. They bring nothing to the table, at all.

For ebooks distributed to libraries... again, I don't think this is a factor? Librarians can weigh in.

PAPERBACKS is where ISBNs matter, because the easiest print option for indies (KDP print) isn't preferred by booksellers and librarians, and I believe you need a purchased ISBN to publish print with IngramSpark.

This is not a first step in self-publishing. This is step ten.

Also, I heard a rumour that IS was going to provide ISBNs to Americans? I'm Canadian, so that doesn't apply to me.

Also also, CANADIAN AUTHORS: we get ISBNs for free through CISS, don't buy them from Bowker. But still, I don't always bother because...unnecessary.

Some countries (Switzerland!) do require ISBNs for proper registration of a published book, but those need to be ones you buy yourself, you can't borrow one from a block someone else controls.